A. Field of Invention
This invention relates to an improvement in the method for preparing para-chlorobenzotrifluoride.
B. Description of the Prior Art
The compound para-chlorobenzotrifluoride (sometimes referred to herein as PCBTF) is used presently for various commercial applications, one of the most valuable being an intermediate in making herbicides, particularly those used on soybeans. As shown in the drawing, PCBTF is conventionally produced by a multi-step process starting with toluene. In this process, toluene is reacted with gaseous chlorine in the presence of iron powder to give ring chlorination which results in a mixture of the ortho-, meta- and para- isomers of chlorotoluene. The orthochlorotoluene is normally the predominantly produced isomer, with the para- isomer produced in a somewhat lesser amount. The meta- isomer is produced in very small amounts, usually from about 10% by weight to about 0.5% by weight of the corresponding para- isomer. In the next step, the ortho- isomer is separated from the meta- and para- isomers, usually by distillation. This separation is easily accomplished because the ortho- isomer has a slightly higher boiling point than the other two isomers, while both the para- and meta- isomers have essentially the same boiling point. Later, the mixture of these latter two isomers are reacted with gaseous chlorine under ultraviolet light to chlorinate the side chain carbon of the chlorotoluene molecule. Then, the resulting mixture of the para- and meta- isomers of chlorobenzotrichloride is fluorinated, usually with excess anhydrous hydrogen fluoride with or without a catalyst, to produce both PCBTF and meta-chlorobenzotrifluoride (sometimes referred to herein as MCBTF). This mixture of PCBTF and MCBTF is recovered together by conventional methods, normally by a phase separation to remove excess unreacted hydrogen fluoride followed by a distillation and condensation of the mixture to separate out any high-boiling residues (e.g., partially fluorinated products).
A problem with the above-described process is that it usually produces MCBTF in sufficient amounts to constitute a substantial impurity to PCBTF. However, in some commercial uses of PCBTF, such as an intermediate in making herbicides, a very low level of impurities such as MCBTF is desirable. And, yet, in each step of the above process, the corresponding meta- and para- isomers have essentially the same physical properties and, therefore, separation of these isomers is very difficult. In fact, all known methods are impractical on large-scale commercial levels.
Therefore, a need exists in the art to develop a way of reducing the amounts of meta- isomer present in PCBTF. Specifically, the present invention accomplishes this by controlling the extent of the above-mentioned fluorination step, thereby producing an improved, commercially acceptable product.